1. Field of the Invention
The present invention relates to a friction stir welding method and a weld structure welded by the friction stir welding method.
2. Description of Related Art
A friction stir welding method is described, for example, in patent document 1 (JP-B2-2712838 (WO93/10935)), the friction stir welding method is structured such as to insert a columnar member (hereinafter, refer to as a rotating tool) made of a substantially harder material than a weld material to a weld portion of the weld material while rotating, and move the rotating tool while rotating, thereby welding by a friction heat generated between the rotating tool and the weld material.
The welding method softens the weld material by the friction heat between the rotating tool and the weld material, utilizes a plastic flow phenomenon coming up with the rotation of the rotating tool, and is based on a different principle from a method of melting the weld material so as to weld, for example, an arc welding or the like.
There is a case that a cavity portion is provided near the weld portion at a time of manufacturing the weld structure having an excellent fatigue strength by using this welding method. An upper portion of the cavity portion is welded by inserting the rotating tool from a perpendicular direction to the weld surface while rotating at a high speed.
In this case, there is not used a backing metal supporting a load at a time of welding. Accordingly, in the case that the weld is executed near the cavity portion, there is a possibility that an unwelded root defect is formed in the weld portion, on the basis of a deformation of the member near the cavity portion.
Therefore, in the case that a stress is applied to the weld portion, an unwelded portion which tends to cause a notch effect is formed, or a stress concentration to the weld portion becomes higher, so that it is considered that fatigue properties are deteriorated. However, the formation of the unwelded portion has not been researched.